1. Field of the Invention
The present invention generally relates to printing devices. The present invention specifically relates to a printing device that ejects an ejecting medium, and to a printing device that mixes and ejects metering and ejecting media. More specifically, this invention relates to a printing device that has the surface of a nozzle member made of polybenzimidazole, which allows high-resolution reproduction of documentary images and improved productivity. Moreover, the invention having the nozzle member made of polybenzimidazole is capable of being manufactured by pressure molding or injection molding, which also promotes improved productivity.
2. Description of the Related Art
Recently, preparation of documents based on computer systems, such as so-called desk top publishing, is favored by clerks in offices. Along with such tendency, a growing need exists among such people for a machine capable of faithfully reproducing images of natural objects in photos together with characters and figures. As a result, a strong demand exists for a high-grade printing device capable of high-quality printing of the images of natural objects. For such printing device to be produced, it is important to reproduce intermediate tones faithfully.
A printer which, only when it receives printing signals, ejects ink droplets through a nozzle onto a printing medium, such as paper and film, or a so-called on-demand type printer have spread quickly in recent years because they can be small in size and produced at a low cost.
A variety of methods have previously been proposed for ejecting ink droplets. However, the methods dependent on the use of a piezoelectric element or a heating element have been generally used. The former consists of ejecting ink under the pressure wrought by a deformed piezoelectric element, and the latter depends on the pressure of bubbles which develops when ink is heated to a boiling point with a heating element.
A variety of methods have also been previously proposed to provide an on-demand type printer, as described above, capable of reproducing intermediate tones faithfully. One such method is to control the size of droplets by adjusting the voltage or width of an electric pulse applied to a piezoelectric element or a heating element, so that the size of printed dots corresponds well with the intermediate tone to be reproduced. With this method, however, if the voltage or width of the pulse delivered to the piezoelectric element or the heating element is chosen too small, ink will not be ejected. Thus, the size of the smallest droplet has a certain limit. This imposes a number of limitations to this method: reproducible tone gradations is limited in number; reproduction of low tones is particularly difficult; and satisfactory reproduction of the images of natural objects can scarcely be achieved.
A second method does not depend on the alteration of dot size. Instead, this method uses pixels each comprising a matrix of 4.times.4 dots and reproduces tones by adjusting the density of pixel depending on the number of excited matrices, or by using the so-called dither method. In this case, each pixel can reproduce 17 different tones. However, when a test pattern with a certain dot density is printed by the two methods, and both printings are compared, the printing by the second method has a resolution one fourth that by the first method. Accordingly, the printing achieved by the second method is too rough to be applied for reproducing the images of natural objects.
In view of this, the present inventors have proposed a printing device capable of faithfully reproducing the images of natural objects without impairing resolution. In this regard, concentration of an ejecting ink droplet can be varied by the addition of diluent to the ink during ejection, so that the printed density of ink can be controlled.
A printing head suitable for such printing device should have a first nozzle member for ejecting medium and a second nozzle member for metering medium placed close to each other. A predetermined volume of the metering medium is pressed out from the second nozzle member towards the first nozzle member to be mixed with the ejecting medium in close vicinity to the orifice of the first nozzle member, so that the ejecting medium can be ejected out together with the metering medium and thereby to effect mixing/ejection of the metering and ejecting media. In the printing device with such printer head, the volume of a metering medium containing either ink or diluent can be varied so that the mixing ratio of ink and diluent can be varied, which enables alteration of dot density. This enables faithful reproduction of the images of natural objects. The metering medium and the ejecting medium can be either ink or diluent; when one is ink, the other is diluent, and vice versa.
A printing device that exercises mixing/ejecting ink and diluent to achieve a faithful reproduction of an image controls the mixing ratio of ink and diluent precisely according to the tone of the image to be printed. For this to be achieved, ink and diluent must be kept separated when they are not mixed or when they are at a stand-by state. If they are in contact with each other while they are at a stand-by state, ink and diluent will diffuse mutually into the other's nozzle: ink to a diluent nozzle and diluent to an ink nozzle. This inadvertent mixture of ink and diluent will gravely affect the mixing ratio of ink and diluent in dots subsequently printed, thereby making it impossible to faithfully reproduce the tone of an image. Accordingly, such printer head will not allow high-resolution reproduction of documentary images. In view of this, providing a space between the orifices of the metering medium nozzle and of the ejecting medium nozzle with a liquid-repellent property is desirable.
This invention is also applicable to a printing device furnished only with an ink nozzle, because adherence of ink around the orifice of the ink nozzle would interfere with smooth ejection of subsequent ink from the orifice so that ink ejection would become instable in its direction. Accordingly, such printer head will not allow high-resolution reproduction of documentary images either.
Adherence of ink around the orifice of the ink nozzle would readily occur in the printing device furnished with the ink and diluent nozzles, unless a liquid-repellent property is conferred to a space between the two nozzles. The liquid-repellent substance previously used for the present purpose generally includes polytetrafluoroethylene or the like. Such substance is applied around the orifices of the nozzles of such printing devices as described above.
As the form of nozzles, particularly of their orifices, gravely affects the direction of liquids ejected from the nozzles, and thus the quality of printed characters, it is conventional to process the nozzles by abrasion with an excimer laser. Abrasion with excimer laser, however, cannot be applied to polytetrafluoroethylene or similar type compounds. To address such inconvenience, a method such as that disclosed in Japanese Unexamined Patent Publication No. 6-328698 is proposed where a material capable of absorbing light whose wave length corresponds to that of an excimer laser is allowed to disperse in polytetrafluoroethylene, and the resulting compound is processed with the excimer laser to prepare a nozzle.
With the method as described in Japanese Unexamined Patent Publication No. 6-328698, however, amenability of a material to processing by abrasion with excimer laser and the liquid-repellency of the processed material can scarcely be compatible: when the former is emphasized, the latter is more or less sacrificed, and vice versa. Further, abrasion with excimer laser, when applied to prepare a nozzle in the considerably thick substance of a film made of polytetrafluoroethylene, can scarcely allow fine processing, which will easily result in development of minute flaws around the processed parts.
Moreover, abrasion with excimer laser is rather complicated in operation: management of gas and the optical system is cumbersome, and large amounts of materials must be consumed in association. These things will contribute to raise the cost for production. In view of this, it is desirable when abrasion with excimer laser is applied for the formation of a nozzle of the printer head of the above-described printing device, to reduce the time necessary for abrasion as much as possible. This will be accomplished by combining injection molding and abrasion with excimer laser, as is disclosed in the above-described Japanese Unexamined Patent Publication No. 6-328698. Specifically, a resin such as polysulfone or the like is subjected to injection molding to produce a film that has a raw form of nozzle formed therein to which a liquid-repellent membrane as described above is applied. Then, abrasion with excimer laser is applied to this assembly, to bore a hole through the polysulfone film and the liquid-repellent membrane. This procedure allows fine working necessary for preparation of the nozzle orifice and its vicinity whose configuration gravely affects the direction the droplet takes when ejected.
When polysulfone or a material having a thermal resistance up to 180.degree. C. is used for this purpose, the material to be used for the formation of the liquid-repellent membrane should polymerize at around 150.degree. C. Further, as described above, the material is necessarily limited to polytetrafluoroethylene having a material dispersed therein that can absorb the light having the same wave length with an excimer laser used for abrasion. These impose severe restrictions on the choice of appropriate materials. This will result in lowered productivity.
Further, if polysulfone is used as a material for the purpose here concerned, because its thermal resistance is rather low, various restrictions will be imposed. For example, a restriction will be imposed on the processes subsequent to the formation of the nozzle, such as bonding of other members onto the nozzle surface. This will likewise result in lowered productivity.
Therefore, a need exists for an improved printer head and methods for preparing same that enable the faithful reproduction of documentary images while also promoting productivity.